Weight Watchers International Inc said Mindy Grossman, current chief executive of HSN Inc., will join the diet company as CEO in July, sending the company's shares up nearly 13 percent in aftermarket trading on Wednesday.

Grossman, who has led HSN since 2008, will also join Weight Watchers' board, the company said in a statement.

Grossman was named Fortune's Business Person of the Year in 2014 and has been named one of the world's 100 most powerful women three times by Forbes magazine, Weight Watchers said.

She has more than 38 years of experience in the consumer industry, including in executive roles at Nike Inc, Polo Jeans Company and Polo Ralph Lauren Corp.

"Mindy is proven as a successful visionary and entrepreneurial force in business and I look forward to working with her," said media mogul Oprah Winfrey, who owns a 10 percent stake in Weight Watchers.

The company said in September last year that the former talk show queen would help it pick a new CEO following James Chambers' resignation after three years on the job.

Weight Watchers' shares were up 12.9 percent at $22.75 in aftermarket trading. They ended up 7 percent in regular trading on Wednesday, including hitting a new 52-week high, bringing their gains for the year to about 76 percent.